1. Field of the Invention
This invention relates to a telemetry system as applied particularly to seismographic exploration.
2. Description of the Prior Art
In seismic exploration, acoustic signals are injected into the earth. The acoustic signals radiate downwardly and are reflected from subsurface formations. The reflected acoustic signals then return to the earth's surface where they are detected by seismic sensors. In marine seismic operations, the seismic sensors are normally included in a cable which may typically extend for more than three miles behind the towing vessel. In the past, it was common to run a separate wire from each sensor, or group of sensors, to the recording apparatus located on the vessel.
U.S. Pat. No. 3,990,036, issued to Savit on Nov. 2, 1986 shows a seismic data telemetry system which utilizes a single data transmission path. A polling signal is transmitted over a single telemetry channel to each sensor. Upon receipt of the polling signal, each sensor in turn transmits data over the same telemetry channel to a recording device. The time delay due to polling signal propagation between successive sensors provides a time window during which each sensor can transmit data.
U.S. Pat. No. 3,996,553 and U.S. Pat. No. 4,023,140 show a system in which a first interrogation signal is transmitted from a central signal processor to data acquisition units on a first interrogation channel. A second interrogation signal is transmitted from the central signal processor on a second interrogation channel at a time which is different from the transmission time of the first interrogation signal. Because the signal propagation velocity is different in the two interrogation channels, the transmission timing may be selected so that the two interrogation pulses will arrive simultaneously at a selected data acquisition unit to initiate transmission of data on a data channel. By this means, each data acquisition unit is commanded to transmit data on the data transmission channel so that the data is time division multiplexed onto the data channel.
A primary disadvantage of telemetering systems known to the prior art is the limited amount of data that can be retrieved.